Electrolytic production of chlorates



Patented Feb. 17, 1953 ELECTROLYTIC PRODUCTION OF CHLORATES William K.Earnest, Portland, reg., and Errol H.

Karr, Tacoma, Wash., assignors to The Pennsylvania Salt ManufacturingCompany, Philadelphia, Pa., a corporation of Pennsylvania.

Application August 12, 1946, Serial No. 689,917

Claims.

This invention relates to improvement in the production of alkali metalchlorates by electrolytic methods.

The improvements of this invention are particularly applicable to theelectrolytic process for the manufacture of metal chlorates as describedin the copending application for United States Letters Patent of ArthurE. Gibbs, Serial No. 674,627, filed on June 5, 1946 (now abandoned).This process involves the use of gaseous chlorine for the acidificationof electrolytes used in the electrolytic production of metal chlorates.Among the advantages of this invention are included the increase in theconcentration of hypochlorous acid caused by the reaction of chlorinewith hydroxyl ions resulting in the acceleration of the formation ofchlorates by reaction between said hypochlorous acid and hypochloriteions, and the conversion of chromate to dichromate ions, by which properacidity in the cell is maintained, without the necessity of addinghydrochloric acid to the cell liquor.

In carrying out the process of the Gibbs application, there are variousprocedures by which chlorine may be introduced into the cell liquor. Forexample-the chlorine may be introduced into the discharge side of a cellfeed pump or into the tank used for making up the electrolyte after thechlorine separation stage. Many of these methods, however, introducedifiiculties in practice. For example, in employing some of the possiblemethods of introducing chlorine, it has been found'advisable to use anexcess of chlorine in order to get the desired effect, and this excesschlorine is later expelled from the cell liquor into the cell room,causing a health hazard.

We have now discovered a simple and effective method and means forintroducing chlorine into the electrolyte of such processes that avoidsthese difiiculties. Our new method and apparatus for introducingchlorine are based on the discovery that, when chlorine is used tocontrol acidity in chlorate cell liquor, it is particularlyadvantageous, in order to make effective use of the chlorine andminimize the health hazard, to provide a seasoning period for thechlorine-treated cell liquor before sending it on to the cells. We havethus found it desirable to season the chlorinetreated cell liquor for atleast 5 minutes after it has first been contacted with chlorine beforesending it on to the cell.

According to a preferred embodiment of my invention the cellelectrolyte, after passing through the chlorate separation stage andhaving been made up to the appropriate composibecomes a predominatefeature.

2 tion by addition of alkali chloride, is introduced in any convenientmanner into a chlorinating tower designed so that the depth ofelectrolyte An inlet for gaseous chlorine and. an outlet for theacidified electrolyte are provided near the bottom of this tower.

indicated time interval to elapse, after the cell liquor has first beentreated with chlorine and before it is sent on to the cells. Thisapparatus also has the advantage of causing the gaseous chlorine to passthrough a substantial body of electrolyte, whereby it is completelyabsorbed, and hence there is no wastage of chlorine. The electrolyteoutlet, being located near the bottom of the tower, has the advantage ofutilizing the pressure resulting from the extreme height of theelectrolyte to cause the electrolyte to flow from the tower throughconnecting lines and into the cells. This eliminates the necessity offeed pumps and the corrosion and other problems inherent thereto.. Afurther advantage is that the reactions initiated by the chlorine havesufficient time to be completed and hence when the electrolyte isintroduced into the cells there is no release of gaseous chlorine to thecell room. A still further advantage of our invention is that thepossibility of the electrolyte backing into the chlorine inlet orcontainers is minimized.

The process and apparatus of our invention are illustrated in theaccompanying drawing, which forms a part of our application. The drawingis both a flow sheet of our preferred process and a diagrammaticrepresentation of our preferred apparatus.

In the drawing the reference numeral I indicates a cylindrical shapedtower which may, for example, be about 21 feet in height and 2 feetinside diameter, fitted with a chlorineinlet 2 opening near the bottomand an electrolyte outlet 3 also near the bottom. The top of the toweris also provided with a vent for spent gas.

The electrolyte enters the chlorinating tower at 4 and is almostimmediately subjected to the action of gaseous chlorine rising in thetower. The acidified electrolyte leaves the chlorinating .tower at 3,after an average stay in the tower .of at least 5 minutes, and flowsthence into the electrolytic cells 5. The conduit 6 communicates with astorage tank I and also the apparatus used for chlorate separation 8.The liquor from 8 runs into a make-up tank 9 where alkali metal chlorideis added. The electrolyte then passes into 1 and is forced :by pump I0into the chlori- This procedure and apparatus have the importantadvantage of permitting the above-.

nating tower l. lhe chlorinating tower also has an overflow line H toreturn excess electrolyte to the storage tank I. We prefer to add theelectrolyte into the top of the tower as shown in the diagram, so that,as the electrolyte flows down towards :the outlet it is subjected toincreasingconcentrations of "gaseous chlorine which is rising in thetower countercurrent to the flow of electrolyte. This method of addingelectrolyte completely eliminates any, stagnation of electrolyte whichmight otherwisebe'presnt in the tower.

In a typical operation involving production of sodium chlorate byelectrolysis of s'odiumchlo-wr ride in aqueous solution, the cellelectrolyte con-- tains both the sodium chloride stai'ti' g material andsome sodium chlorate product irom previous operation. The electrolytealso contains a small amount of sodium dichrornate as a curler materialto maintain the desired pH of 6.7 to 6.9 in the ,ce1ls:-

example, in a typical electrolyte; there may be 150 grams sodiumchloride per liter, 350 grams sodium chlorate, and 5 grams sodiumdichromate. as electrolysis goes on and theum chloride is converted tosodium chlorate, the p H of the cell liquor tends to rise. However, inthe presence of sodium dichrornate the alkaline materials tending. toraise the cell liquor pl-l react with the dichromateconvert it tochromate, and the cell pH isthus at least partially stabilized.

,.In, order to maintain proper p21, cell ii nor, in which a substantialpart c L matehas thus been convei a pH of about 6.9 and is constantlybeing: r by cell liquor of pH about 5.8 to 6 in win 1 "lie chromate hasbeen at least in part reconverted to dichromate by action of gaseouschlorine. During this recirculating, some sodium chlo ate is alsoremoved from the cell honor, as product, and the cell liquor isfortified sodium chl ride, as pointed out in connection with thedrawing.

Since many modifications are possible in the process and apparatus ofour invention as above described without departing from the. scope ofthe invention, it is intended the above description of our inventionshould be interpreted as illustrative, and the invention is not, to belimited except as set forth in the which follow:

We claim:

1. In a process for the elec of an alkali metal chlorate by an acidicaqueous solution chloride, employing "eous c taining properacid on thesciatic step that comprisesnoicmgthe-solution least five niinutes afternrst contacting the acidic solution \vith chlorine and beforeintrooucingit into the electrolytic cell. I

.2. .Therproces's for production of sodium chic-- rate thatcomprisessubjecting an "acidic sodium chloride solution to electrolysisfor electrolytic conversion of the chloride to chlorate, treating theacidic electrolyte with elemental chlorine priOtJlO itsintroduction intothe electrolysis-zone,

4 to counteract the tendency of the electrolyte to diverge from a setrange of acid concentration, and holding the treated electrolyte for atleast five minutes after first contacting it with chlorine and beforeintroducing it into the electrolytic cell. j

The process for production of alkali metal chlorate that comprisessubjecting an acidic alkali metal chloride solution to electrolysis forelectrolytic conversion of the chloride to chlorate, saidelectrolytecontaining also a small amount of dichromate in solution as a bufferingagent, treating the acidic electrolyte with elemental chlorine:ata'polnt outside the electrolytic cell to counter act' divergence ofthe electrolyte from a set range of acid concentration, and holding thetreated electrolyte for at least five minutes after first contacting itwith chlorine and before introducing it intolthe electrolytic cell.

l. The cyclic process for production of sodium chloratethat comprisessubjecting a sodium chloride solution of pH 5.8 to 6.9 to electrolysisfor electrolytic conversion of the chloride to chlorate,said-electrolyte containing-alco e small amount of dichromate insolution as a buffering agent, withdrawing, electrolyte from the cell,seoarating sodium chlorate therefron treating the acidic electrolyte,while outsidethe ,cell, with elemental chlorine, to counteract tl iecivei ence of the, electrolyte from the desiredval uein said pH range,holding thetreated electrolyte iorat least five minutes after firstcontacting it with chlorine, and thereafter reintroducing the resultingelectrolyte into the cell.

5. The cyclic process for production of sodium chlorate that comprisessubjecting a sodium chloride solution or pH not greater 'thanififi toelectrolysis for electrolytic conversion of the chloride tochlorate; tif ing electrolyte from the cell, separating sodium chlorate therefrom,treating the lectrolyte of pH not greater than 6.9, While outside thecell, thelemental chlorine, maintainingthe acctrolyte chic-rinejincontact ehas been absorbed and hydrolyzed to lower the pI-I oi, thetreated electrolyte appreciably, and thereafter reintroducing thetreatedelctrolyte intofthe cell. i. i E- ILLIARE K. EARNEST. .Q

REFERENCES crash The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES The Alkali Industry, byPartington (1925) page 186.

o at least minutes and untllsunicient chlo

1. IN A PROCESS FOR THE ELECTROLYTIC PRODUCTION OF AN ALKALI METALCHLORATE BY THE ELECTROLYSIS OF AN ACIDIC AQUEOUS SOLUTION OF AN ALKALIMETAL CHLORIDE, EMPLOYING GASEOUS CHLORINE FOR MAINTAINING PROPERACIDIFICATION OF THE SOLUTION, THE STEP THAT COMPRISES HOLDING THESOLUTION, THE LEAST FIVE MINUTES AFTER FIRST CONTACTING THE ACIDICSOLUTION WITH CHLORINE AND BEFORE INTRODUCING IT INTO THE ELECTROLYTICCELL.